Aqueous viscoelastic fluid

ABSTRACT

The invention concerns an aqueous viscoelastic fluid for use in the recovery of hydrocarbons. According to the invention this fluid comprises a first viscoelastic surfactant and a second surfactant able to decompose under downhole conditions to to reduce the viscosity of the aqueous viscoelastic fluid.

[0001] The present invention concerns an aqueous viscoelastic fluid foruse in the recovery of hydrocarbons and, in particular, for use as afracturing fluid.

BACKGROUND OF THE INVENTION

[0002] Hydrocarbons such as oil or natural gas are obtained fromhydrocarbon-bearing subterranean geologic formations via flow pathsconnecting a reservoir of said formations and the wellbore. Impeded flowpaths may lead to an insufficient hydrocarbon production. In such case,various techniques are used to stimulate this production. Amongst thesetechniques, it is common to inject specialised fluids via the wellboreinto the formation at sufficient pressures to create fractures in theformation rocks through which the hydrocarbons may more readily flowinto the wellbore. The latter technique is referred to as fracturing orhydraulic fracturing and the specialised fluids used in said techniqueare referred to fracturing fluids.

[0003] Ideally, fracturing fluids should impart a minimal pressure dropin the pipe within the wellbore during placement and have an adequateviscosity to carry a propping agent that prevents the fracture fromclosing. Also, they should have a minimal leak-off rate and shoulddegrade so as not to leave residual material that may prevent accuratehydrocarbons to flow back into the wellbore.

PRIOR ART

[0004] Aqueous fracturing fluids wherein the gelling agent is aviscoelastic surfactant have been developed and commercialised. They aredisclosed notably in the patents published under the numbers U.S. Pat.No. 4,695,389, U.S. Pat. No. 4,725,372 and U.S. Pat. No. 5,551,516. Anexample of such fluid is commercialised by the company groupSchlumberger™ under the trademark ClearFRAC™. It is a mixture of aquaternary ammonium salt, N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride, with isopropanol and brine, said brine typicallyincluding water and either 3% by weight of ammonium chloride or 4% byweight of potassium chloride. In such fluids, surfactant molecules,present at a sufficient concentration, aggregate into overlapping worm-or rod-like micelles. This confers a sufficient viscoelasticity to saidfluids for carrying the propping agent. At very high shear rate however,in particular above 170 s⁻¹, the viscosity falls drastically. Thisallows the fluid to be pumped down the wellbore. Also, the worm- orrod-like micelles aggregates tend to break by contact with hydrocarbons.So, if no surfactant emulsion is effectively formed, the surfactantmolecules are normally carried along the fracture to the well boreduring hydrocarbon backflow.

[0005] Under certain circumstances, for example when fracturing dry gasreservoirs wherein negligible quantities petroleum gas condense duringproduction, the breaking of the gel can be hindered by the absence ofany significant quantities of liquid hydrocarbon in the produced fluids.As a result, the efficiency with which the fracturing fluid is removedfrom the propped fracture is reduced.

[0006] That is one of the reasons why it has been proposed to adddelayed breakers to viscoelastic fracturing fluids. These delayedbreakers are able to break the fluid gel structure and reduce itsviscosity at an appropriate time after the fracturing operation per se.

[0007] Delayed breakers of aqueous viscoelastic fluids comprisingviscoelastic surfactant have been disclosed in the application publishedunder the number WO-01/77487. They can be external or internal breakers.

[0008] External breakers are initially isolated from the surfactantmolecules of the fluid. Typically, they consist of a solid materialsuspended and transported by this fluid as it creates the proppedfracture. The solid material has generally a core-shell structure wherethe core is the chemical which breaks the gel and the shell is anencapsulating material which isolates the core from the gel. At anappropriate time within the propped fracture, the shell materialdissolves, decomposes or ruptures and the core material breaks the gel.

[0009] Internal breakers are compounds which are initially dissolvedwithin the fluid and are not isolated from the surfactant molecules. Atan appropriate time, they decompose to release degradation productswhich break the gel. In the above-referenced application WO-01/77487, itis taught that the viscosity of an aqueous viscoelastic gel comprisingviscoelastic surfactants consisting of long chain quaternary ammoniumsalts is reduced by the addition of esters. Esters have by themselves alittle effect on the initial gel rheology. However, they can decomposeto release alcohols that decrease the gel viscosity.

[0010] The gel breaking efficiency of alcohols increases with theirconcentration in the gel, the temperature and, also, with the molecularweight of said alcohols. However, the compatibility of esters with theviscoelastic surfactant based gel decreases with their hydrophobicity.As the molecular weight of alcohols is proportional to thehydrophobicity of the esters, then the ester approach is limited by therelationship between the hydrophobicity of the esters and theircompatibility with the gel.

SUMMARY OF THE INVENTION

[0011] Considering the above prior art, one problem that the inventionis proposing to solve is to carry out an aqueous viscoelastic fluid foruse in the recovery of hydrocarbons and, in particular, for use as afracturing fluid, said fracturing fluid comprising a compatible internalbreaking system able to release efficient breaker compounds.

[0012] As a solution to the above problem, the invention concerns, in afirst aspect, an aqueous viscoelastic fluid for use in the recovery ofhydrocarbons, comprising: a first surfactant, said surfactant beingviscoelastic; and a second surfactant, said second surfactant being ableto decompose under downhole conditions to release a compound, saidcompound being able to reduce the viscosity of the aqueous viscoelasticfluid.

[0013] In a second aspect, the invention concerns a method for use inthe recovery of hydrocarbons comprising the following steps: providingan aqueous viscoelastic fluid comprising a first surfactant, saidsurfactant being viscoelastic, and a second surfactant able to decomposeunder downhole conditions; allowing said second surfactant to decomposeunder downhole conditions to release a compound able to reduce theviscosity of the aqueous viscoelastic fluid; and allowing the viscosityof the fluid to be reduced downhole.

[0014] The second surfactant is, as the first surfactant, amphiphilic.It has a hydrophilic head group and a hydrophobic tail group. It iscompatible with the first surfactant and may even participate in theformation of the viscoelastic gel. Under certain conditions or/and aftera certain time, it decomposes to release degradation products, one ofthese degradation products being a compound able to reduce the viscosityof the viscoelastic gel and break this gel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will be better understood in the light of thefollowing description of non-limiting and illustrative embodiments givenwith reference to the accompanying drawings, in which:

[0016] the FIG. 1 shows the breakdown reaction of erucyl ester methylenedimethyl ethyl ammonium chloride;

[0017] the FIG. 2 shows the breakdown reaction of mono-oleyl succinate;

[0018] the FIG. 3 shows the breakdown reaction of disodium laurethsulphosuccinate;

[0019] the FIG. 4 shows the breakdown reaction of sodium laurylsulphoacetate;

[0020] the FIG. 5 illustrates the effect alcohols on rheology ofN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride based gels;

[0021] the FIG. 6 illustrates the effect of butanol concentration andtemperature on rheology of N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride based gels;

[0022] the FIG. 7 illustrates the effect of oleyl alcohol on theviscosity of N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloridebased gels;

[0023] the FIG. 8 illustrates the impact of the hydrophobicity of estersto the compatibility of N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride based gels;

[0024] the FIG. 9 illustrates the effect of erucyl ester methylenedimethyl ethyl ammonium chloride onN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride based gels;

[0025] the FIG. 10 compares the rheology of gels comprising erucyl estermethylene dimethyl ethyl ammonium chloride and N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride;

[0026] the FIG. 11 illustrates the effect of temperature on the rheologyof a gel comprising N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammoniumchloride and a diesterquat;

[0027] the FIG. 12 illustrates the effect of temperature on the breakingtime of a gel comprising N-erucyl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride and a diesterquat;

[0028] the FIG. 13 illustrates the effect of oleyl alcohol concentrationon the rheology of a gel based on dimer oleic acid;

[0029] the FIG. 14 compares the low shear viscosity of gels based ondimeric oleic acid as a function of chloride concentration and fluid pH;

[0030] the FIG. 15 compares the low shear viscosity at salt peak offluids of FIG. 14 as a function of temperature and fluid pH;

[0031] the FIG. 16 illustrates the delayed breakdown of viscoelasticsurfactant gels based on dimeric oleic acid in the presence of theinternal cleavable surfactant breaker mono-oleyl ester succinate;

[0032] the FIG. 17 illustrates the effect of asulphosuccinate/sulphoacetate mixture and sulphosuccinate on flowrheology of a gel system based on dimeric oleic acid;

[0033] the FIG. 18 illustrates the delayed breaking of a dimeric oleicacid based viscoelastic gel dosed with a sulphosuccinate/sulphoacetatecleavable surfactant mixture;

[0034] the FIG. 19 illustrates the fact that the breaker dosage can beused to control the gel degradation rate;

[0035] the FIG. 20 shows the linear relationship that exists between theconcentration of active surfactant and the inverse of the time requiredfor a gel to lose 90% of its viscosity at low shear rate;

[0036] the FIG. 21 illustrates the effect of temperature on of a dimericoleic acid based viscoelastic gel comprising asulphosuccinate/sulphoacetate cleavable surfactant mixture; and

[0037] the FIG. 22 compares the gel breakdown kinetics for of a dimericoleic acid based viscoelastic gel dosed with asulphosuccinate/sulphoacetate mixture or sulphosuccinate cleavablesurfactants.

DETAILED DESCRIPTION

[0038] The present invention concerns an aqueous fluid for use in therecovery of hydrocarbons such as oil and gas. This aqueous fluid is awellbore service fluid such as a drilling fluid, a completion fluid, awork over fluid, a packer fluid or a conformance or permeability controlfluid and, more particularly, a fracturing fluid.

[0039] The fluid of the invention is viscoelastic. Its viscoelasticitymay be measured by carrying out dynamic oscillatory rheologicalmeasurements as generally described in Barnes H. A. et al., AnIntroduction to Rheology, Elsevier, Amsterdam (1997). In a typicaldynamic oscillatory experiment, the fluid is sheared sinusoidallyaccording to the following equation (1):

γ(t)=γ_((max))sin ωt  (1)

[0040] where γ(t) is the strain, γ(max) is the maximum strain, t is timeand ω is the angular frequency. The shear stress, σ, is given by:

σ(t)=σ_((max))sin(ωt+δ)  (2)

[0041] where δ is the phase angle. The relative inputs given by theelastic component (G′) and viscous component (G″) are resolved asfollows. Expanding the sine function in equation (2) gives equations (3)and (4) as follows:

σ(t)=σ_((max))[sin ωt cos δ+cos ωt sin δ]  (3)

σ(t)≡γ[G′ sin ωt+G″ cos ωt]  (4)

[0042] where G′≡(σ_((max))/γ_((max))) cos δ and G″≡(σ_((max))/γ_((max)))sin δ.

[0043] Equation (4) therefore defines two dynamic moduli: G′, thestorage modulus or elastic component and G″, the loss modulus or viscouscomponent of a fluid having viscoelastic properties.

[0044] The fluid of the present invention is an aqueous viscoelasticgel, where the terms “viscoelastic gel” as used herein mean acomposition in which the elastic component (G′) is at least as importantas the viscous component (G″). In the evolution from a predominantlyviscous liquid to a viscoelastic gel, the gel point can be defined bythe time when the contribution from the elastic and viscous componentsbecomes equal, i.e. G′=G″; at and beyond this point in time, G′≧G″ andthe phase angle, δ is ≧45°.

[0045] The fluid of the invention comprises a first surfactant. Thissurfactant is said viscoelastic because, unlike numerous surfactantswhich typically form Newtonian solutions with a viscosity slightlyhigher than water even at high concentration, it is capable of formingviscoelastic fluids even at lower concentrations. This specificrheological behaviour is mainly due to the types of surfactantaggregates that are present in the fluids. In the fluids with lowviscosity, the surfactant molecules, present at a sufficientconcentration, aggregate in spherical micelles whereas, in viscoelasticfluids, long micelles, which can be described as worm- or rod-likemicelles, are present and entangle.

[0046] The first surfactant of the invention is usually ionic. It may becationic, anionic or zwitterionic depending on the charge of its headgroup. When the surfactant is cationic, it is associated with a negativecounterion which is generally Cl⁻ or an anionic organic species such thesalicylate anion. When the surfactant is anionic, it is associated witha positive counterion, generally Na⁺ or K⁺ and, when it is zwitterionic,it is associated with both negative and positive counterions, generallyCl⁻ and Na⁺ or K⁺.

[0047] The first surfactant is, for example, of the following formulae:

R-Z

[0048] where R is the hydrophobic tail of the surfactant, which is afully or partially saturated, linear or branched hydrocarbon chain of atleast 18 carbon atoms and Z is the head group of the surfactant whichcan be —NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or, in the case where the surfactant iszwitterionic, —N⁺(R₁R₂R₃—COO⁻) where R₁, R₂ and R₃ are eachindependently hydrogen or a fully or partially saturated, linear orbranched, aliphatic chain of at least one carbon atom, possiblycomprising a hydroxyl terminal group.

[0049] In another example, the first surfactant is a cleavableviscoelastic surfactant of the following formulae:

R—X—Y-Z

[0050] where R is the hydrophobic tail of the surfactant, which is afully or partially saturated, linear or branched hydrocarbon chain of atleast 18 carbon atoms, X is the cleavable or degradable group of thesurfactant which is an acetal, amide, ether or ester bond, Y is a spacergroup which is constituted by a short saturated or partially saturatedhydrocarbon chain of n carbon atoms where n is at least equal to 1,preferably 2 and, when n is ≧3, it may be a straight or branched alkylchain, and Z is the hydrophilic head group of the surfactant which canbe —NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or, in the case where the surfactant iszwitterionic, —N⁺(R₁R₂R₃—COO⁻) where R₁, R₂ and R₃ are eachindependently hydrogen or a fully or partially saturated, linear orbranched, aliphatic chain of at least one carbon atom, possiblycomprising a hydroxyl terminal group.

[0051] A cationic viscoelastic surfactant suitable for theimplementation of the invention is theN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride. In anaqueous solution comprising 4 wt % NaCl or 3 wt % KCl, this viscoelasticsurfactant forms a gel containing worm-like micelles that entangle atconcentrations typically in the range 1-10 wt %. These worm-likemicelles degrade to form spherical micelles when the gel is broken byhydrocarbons.

[0052] Anionic viscoelastic surfactants suitable for the implementationof the invention are monocarboxylates RCOO⁻ such as oleate where R isC₁₇H₃₃ or di- or oligomeric carboxylates such as disclosed in the patentapplication filed on the 11 Jul. 2001 under the number PCT/GB01/03131not published at the filing date of the present patent application.These mono-, di- or oligomeric carboxylates form viscoelastic gels whenin alkaline solution in the presence of added salts such as potassiumchloride or sodium chloride. Worm-like micelles of said gel degrade tospherical micelles when the gel is broken by hydrocarbon.

[0053] The fluid of the invention comprises a second surfactant. Thissurfactant is viscoelastic or not. It is said cleavable. As such, itdecomposes under downhole conditions to release degradation products.Cleavable surfactants for the implementation of the invention aredisclosed in the patent application filed on the 13 Feb. 2001 under thenumber GB 0103449.5. These surfactants are viscoelastic of the followingformulae:

R—X—Y-Z

[0054] where R is the hydrophobic tail of the surfactant, which is afully or partially saturated, linear or branched hydrocarbon chain of atleast 18 carbon atoms, X is the cleavable or degradable group of thesurfactant which is an acetal, amide, ether or ester bond, Y is a spacergroup which is constituted by a short saturated or partially saturatedhydrocarbon chain of n carbon atoms where n is at least equal to 1,preferably 2 and, when n is ≧3, it may be a straight or branched alkylchain, and Z is the hydrophilic head group of the surfactant which canbe —NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or, in the case where the surfactant iszwitterionic, —N⁺(R₁R₂R₃—COO⁻) where R₁, R₂ and R₃ are eachindependently hydrogen or a fully or partially saturated, linear orbranched, aliphatic chain of at least one carbon atom, possiblycomprising a hydroxyl terminal group.

[0055] Typical second surfactants are therefore ester carboxylates,ester sulphonates, for example, where Y═CH₂CH₂, isethionates, and esterquats. The equivalent reverse and forward amide surfactants, that is tosay reverse amide carboxylates, forward amide carboxylates, for examplesarcosinates (RCON(CH₃)CH₂COO⁻), reverse amide sulphonates, forwardamide sulphonates, for example taurates (RCON(R′)CH₂CH₂SO₃ ⁻), reverseamide quats and forward amide quats are also typical second surfactantsaccording to the invention.

[0056] Due to, in particular, the presence of the hydrophilic headgroup, weight percent concentrations of R—X—Y-Z surfactants arecompatible with the viscoelastic surfactant gel even when R is asaturated or partially unsaturated chain with 18 or more carbon atoms.

[0057] For example, when X is an ester group, the cleavable surfactantis therefore able to decompose under downhole conditions to release analcohol breaker according to the following reaction:

ROOC—Y-Z+OH⁻→⁻OOC—Y—X+ROH.

[0058] In the same way, the hydrolysis of reverse amide surfactantsgenerates amines which are also an efficient breakers. Also, thehydrolysis of forward ester or forward amide surfactants generatescarboxylic acids which can also be efficient gel breakers, in particularwhen the first surfactant is cationic.

[0059] Typically, the alcohol, the amine and the carboxylic acidsgenerated comprise at least 3 carbon atoms. Preferably, they are longchain alcohol, amine or carboxylic acid comprising 8 to 18 carbon atomsor more.

[0060] Finally, internal delayed breakers based on cleavable surfactantsare advantageously selected according to the invention such that theymeet the following performance criteria:

[0061] to be present at a concentration enough to generate a sufficientquantity of gel breaker compound, notably a long alcohol, amine orcarboxylic acid, without degrading the initial rheological properties ofthe viscoelastic surfactant gel and, preferably, with enhancing theproperties of said viscoelastic surfactant gel; and

[0062] to degrade, at said concentration, at a controllable rate whichis appropriate for a given application. For fracturing application, geldegradation should be controllable in the following range from 1 to 5hours.

[0063] The pH of the viscoelastic gels based onN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride may be nearneutral when formulated with potassium chloride and mildly acidic whenformulated with ammonium chloride. Since the cationic surfactantmaintains its positive charge and gelling properties through a broadrange of acid, neutral and alkaline conditions, there is scope to usecationic cleavable surfactant breakers in which the cleavable linkage isan ester or amide.

[0064] Esterquats are the preferred cleavable surfactant breakers forsuch gels. Their chemistry, properties and uses are disclosed in KrugerG., Boltersdorf D. and Overkempe K., “Estequats”, Novel Surfactants:Preparation, Applications & Biodegrability, edited by Krister Holmberg,Marcel Dekker, Inc., New York, 1998, pp. 114-138. The general formulaefor mono-esterquats is R—COO—C_(n)—N(R)₃ ⁺ for a forward ester andR—OOC—C_(n)—N(R)₃ ⁺ for a reverse ester where, typically, n is 1, 2 or 3and preferably 2. Usually, they are prepared by reacting a tertiaryalkanolamine with a fatty acid, followed by reaction with an alkylatingagent to the corresponding quaternary as disclosed in PCT applicationpublished under the number WO-91/01295. For example, mono- anddi-esterquats may be prepared according to the following reactions:

(CH₃)₂N(CH₂)₂OH+RCOOH→(CH₃)₂N(CH₂)₂OOCR+H₂O and

(CH₃)₂N(CH₂)₂OOCR+CH₃Cl→(CH₃)₃N⁺(CH₂)₂OOCR Cl⁻

[0065] and

CH₃N((CH₂)₂—OH)₂+2RCOOH→CH₃N((CH₂)₂—OOCR)₂+2H₂O and

CH₃N((CH₂)₂—OOCR)₂+CH₃Cl→(CH₃)₂N⁺((CH₂)₂—OOCR)₂Cl⁻

[0066] Less common esterquats are derived from sugar derivatives,wherein the sugar is incorporated via esterification of a carboxylicacid or hydroxyl group. In particular, example of esterquats derivedfrom glucose or sorbitol are described in Kationische Zuckertenside,Seifen Oele Fette Wachse 120:423, 1994. Other examples of esterquatsderived from sorbitol are described in the German Patent published underthe number 195 39 876. Also, examples of esterquats derived fromgluconic acid are given in the German Patent published under the number195 39 845.

[0067] Other esterquats are betaine esters which derive fromaminocarboxylic acids and thus have a reverse ester group compared tothe forward esterquats based on alkanolamines. Such betaine esters aredisclosed in the documents Biermann M., Lange F., Piorr R., Ploog U.,Rutzen H., Schindler J. and Schmidt R., Surfactants in Consumerproducts, edited by J. Falbe, Springer-Verlag, Heidelberg (1987), pp.110-114 and Edebo L., Lindstedt S., Allenmark S. and Thompson R. A.,Antimicrob. Agents Chemother. 34:1949 (1990).

[0068] Esterquats with two different ester bonds, R—COO⁻ and R—OOC, inthe same molecule are disclosed in the application published under thenumber WO 93/17085. They are prepared by reacting dimethyl ethanolaminewith fatty acid and subsequent quaternisation with alkylchloroacetate.

[0069] One manufacturer of esterquats is Akzo Nobel™ and the productrange of esterquats commercialized by Akzo Nobel™ is marketed under thename Armosoft™. Another manufacturer is Stepan™. This manufacturermarkets suitable products under the names AMMONYX GA-90™ and AMMONYXGA-70PG™ which contain the diesterquat shown below:

[0070] This diesterquat is di(palmitoylethyl)hydroxyethylmethylammonium. The counterion is methosulfphate CH₃OSO₃ ⁻.AMMONYX GA-90™ comprises 90 wt % of the diesterquat and 10 wt % ofisopropanol whereas the AMMONYX GA-70PG™ comprises 70 wt % of thediesterquat and 30 wt % propylene glycol.

[0071] Another esterquat suitable for the implementation of theinvention is erucyl ester methylene dimethyl ethyl ammonium chlorideshown in the following formula:

[0072] Under certain conditions, the reverse ester bond of thisesterquat cleaves resulting in the generation of erucyl alcoholaccording to the breakdown reaction shown in FIG. 1. Erucyl alcohol isan efficient breaker of aqueous viscoelastic fluids of the invention.

[0073] Viscoelastic gels comprising oleate surfactants require analkaline condition with a pH equal or greater than about 11. Given thisconstraint, candidate internal delayed breakers are a broad range ofanionic cleavable surfactants including:—esters, amides or ethercarboxylates;—ether sulphonates;—ether sulphates; and—phosphate esters.Their suitability however depends on their ability to deliver theappropriate degradation kinetics starting from an initial pH equal orgreater than about 11.

[0074] A cleavable surfactant suitable for the oleate surfactantviscoelastic gels is the mono-oleyl ester succinate. It is an anioniccleavable surfactant comprising a cleavable ester bond between the oleylhydrophobic and the carboxylate hydrophilic group. Under alkalineconditions, it cleaves to release oleyl alcohol and the succinate anion.The corresponding reaction is shown in the FIG. 2.

[0075] Other cleavable surfactants may however be suitable for breakingoleate surfactant gels or dimer/trimer carboxylate gels. These are basedon sulphosuccinate and sulphoacetate surfactants.

[0076] For example, alkyl sulphosuccinates are mono- or di-esters ofsulphosuccinic acid HOOCCH₂—CH(SO₃H)COOH. The formulae of these mono-and di-esters of sulphosuccinic acid are as follows:

ROOCCH₂—CH(SO₃Na)COONa (monoester)

ROOCCH₂—CH(SO₃Na)COOR (diester)

[0077] where R is an alkyl chain.

[0078] Two different second surfactants suitable for the implementationof the invention are available from the Stepan™ Company. The first isdisodium laureth sulphosuccinate:

[0079] The second is a sulphoacetate surfactant, sodium laurylsulphoacetate:

[0080] STEPAN-MILD LSB™ is a liquid product containing both the disodiumlaureth sulphosuccinate and sodium lauryl sulphoacetate surfactants inwater, the total surfactant activity being 25 wt %. This surfactanttolerates hard water and it is readily biodegradable. The recommendedtemperature for storage is between 7° C. and 43° C. STEPHAN-MILD SL3™ isalso a liquid containing 30 wt % disodium laureth sulphosuccinate. Bothsurfactants can decompose to release long chain alcohols as illustratedin the FIGS. 3 and 4. This decomposition is accompanied by a decrease influid pH due to the consumption of the ion OH⁻. In addition, thepresence of the sulphonate group accelerates the rate of esterhydrolysis such that ROOCCH₂—CH(SO₃Na)COONa degrades more rapidly thanits non-sulphonated equivalent ROOCCH₂—CH₂COONa and ROOCCH₂—SO₃Na willdegrade more rapidly than ROOCCH₃. In both cases, the presence of thesulphonate group increases the hydrophilicity and water solubility ofthe compound and this enhances compatibility with the gel.

[0081] In addition to the first and second surfactants, the aqueousfluid of the invention may comprise salts including, for example,inorganic salts such as ammonium, sodium or potassium chlorides presentin concentrations of 1 to 10 wt % and, typically, 3 to 4 wt %, ororganic salts such as sodium salicylate. The fluid may also comprise anorganic solvent such as isopropanol, which increases the liquefaction ofthe surfactant molecules.

[0082] Practically, all compounds of the fluid of the invention areblended at surface together with the propping agent, which can be, forexample, a 20-40 mesh sand, bauxite or glass beads. When subjected to avery high shear rate, the viscosity of the fluid is sufficiently low toallow its pumping downhole. There, the pumped fluid is injected into theformation rocks to be fractured under a high pressure. At that time, thefluid of the invention is sufficiently viscous for carrying the proppingagent through the fracture. At a given time after fracturing per se, thesecond surfactant decomposes to release a compound that will break thegel. This appears particularly advantageous when the producedhydrocarbons flowing back the fractures is substantially free ofsignificant quantity of hydrocarbon in a liquid phase.

EXAMPLE 1 Effect of Alcohols on the Fluid Rheology

[0083] On FIG. 5 is plotted the viscosity of a gel comprising 3 wt %N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride, 1 wt %isopropanol and 3 wt % NH₄Cl and that of equivalent gels which alsocontain 1 wt % methanol, ethanol, n-propanol, isopropanol, n-butanol orn-pentanol, as a function of shear rate, at 60° C. The presence of a lowconcentration of alcohol reduces the viscosity of the viscoelasticsurfactant gel. In particular, the viscosity is reduced at a shear ratebelow 10 s⁻¹. The gel breaking efficiency increases with the number ofcarbon atoms in the alcohol and so, with the hydrophobicity of saidalcohol.

[0084] On FIG. 6 is plotted the viscosity of an aqueous gel comprising 3wt % of a fluid (cationic VES) comprising 75 wt %N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride and 25 wt %isopropanol, 3 wt % NH₄Cl and 0, 0.5, 1 or 1.5 wt % butanol, as afunction of shear rate, either at 25 or at 60° C. As shown in thisfigure, the gel breaking efficiency of alcohols also increases with thealcohol concentration and with temperature.

[0085] On FIG. 7 is plotted the viscosity of an aqueous gel comprising 2wt % of a fluid comprising 60.5 wt %N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride, isopropanoland ethylene glycol, 3 wt % KCl and oleyl alcohol, as a function of theoleyl alcohol concentration, at room temperature, under a low shear rateof . . . s⁻¹. As shown in this figure, the addition of oleyl alcoholcauses a dramatic decrease in the low shear viscosity of the gel whichincreases with its concentration.

EXAMPLE 2 Relationship Between the Hydrophobicity of Esters andCompatibility with the Fluid

[0086] On FIG. 8 is plotted the viscosity of an aqueous gel containing4.5 wt % surfactant (comprising 75 wt %N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride and 25 wt %isopropanol), 0.75 wt % hydrophobically-modified polyacrylamide, 3 wt %NH₄Cl and a dimethyl dibasic ester which can be dimethyl itaconate,dimethyl malonate, dimethyl malate, dimethyl oxalate, dimethylglutarate, dimethyl adipate, dimethyl malonate or dimethyl azelate, as afunction of the dibasic ester concentration, at 25° C. The morehydrophilic dibasic esters, for example dimethyl itaconate, dimethylmalate and dimethyl oxalate are compatible with the gel even whenpresent at 3-4 wt %.

[0087] The alkaline hydrolysis of dibasic esters is described by thefollowing reaction:

R₂OOC—Y—COOR₂+2OH⁻→⁻OOC—Y—COO⁻+2R₂OH

[0088] where R₂ are alkyl groups and Y is a link group in the dibasicester.

[0089] In the present example, R₂ is CH₃ and Y depends on the particulardibasic ester chosen. It appears that, as the number of carbon atomsincreases in Y and so, as the hydrophobicity of the dibasic esterincreases, its compatibility with the viscoelastic surfactant gel isreduced. The gel compatibility limit determined from the FIG. 8 is givenby the addition of about 1 wt % of dimethyl glutarate, which candecompose to generate 0.4 wt % methanol.

[0090] This relationship between the hydrophobicity of esters andcompatibility with the fluid would have been the same for classicalmonobasic esters which hydrolyses under alkaline conditions according tothe following reaction:

R₁COOR₂+OH⁻→R₁COO⁻+R₂OH

[0091] where R₁ is also an alkyl group.

EXAMPLE 3 Aqueous Viscoelastic Fluid Wherein the Second Surfactant isErucyl Ester Methylene Dimethyl Ethyl Ammonium Chloride

[0092] On FIG. 9 is plotted the viscosity of aqueous gels comprising 2wt % of a fluid (cationic surfactant) comprising 60.5 wt %N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride, isopropanoland ethylene glycol, 4 wt % KCl and 0 or 0.5 wt % erucyl ester methylenedimethyl ethyl ammonium chloride as a function of shear rate, at roomtemperature.

[0093] It appears that erucyl ester methylene dimethyl ethyl ammoniumchloride is compatible with a typical theN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride fluid. Itspresence even actually enhances the initial viscosity of the gel.

[0094] The degradation kinetics of the above gel comprising 0.5 wt %erucyl ester methylene dimethyl ethyl ammonium chloride was then studiedfor various pH and at 25, 45 or 60° C. Table 1 below illustrates theresults that were obtained: TABLE 1 Degradation Time Degradation Time(hours) (hours) T Initial pH [Time to <1000 cP [Time to <50 cP at (° C.)pH control at 1 s⁻¹] 100 s⁻¹] 25 6.32 0.5 wt % 7.5 20 NH₄ acetate 258.46 0.1% K 3 3 bicarbon- ate 45 5.17 0.5 wt % K >12 16 acetate +CH₃COOH 45 6.32 0.5 wt % 2.5 9 NH₄ acetate 60 7.99 0.1 wt % K 1 1acetate 60 7.47 0.5 wt % K 2.8 2.8 formate 60 7 No 7.5 7.5 buffer(evolves to acid pH) 60 6.32 0.5 wt % 0.8 1.2 NH₄ acetate 60 7 No 3 3buffer* (evolves to acid pH)

[0095] By varying the initial pH of the fluid using simple bufferadditives, it is possible to delay the gel breaking process from 1 to 24hours. A longer delay is achieved when a more acidic conditions is used.Near-neutral or mildly alkaline condition is appropriate for lowtemperature treatments comprised between 25 and 45° C. Near-neutral ormildly acidic condition is appropriate for higher temperature rangecomprised between 45 and 60° C.

[0096] On FIG. 10 is plotted the viscosity of aqueous gels comprising 2wt % of a fluid (cationic surfactant) comprising 60.5N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride, isopropanoland ethylene glycol, 4 wt % KCl and 0.5 wt % erucyl ester methylenedimethyl ethyl ammonium chloride as a function of shear rate, fordifferent times, at 60° C. In an initial phase from 0 to 70 minutes, thepartial breakdown of erucyl ester methylene dimethyl ethyl ammoniumchloride results in an increase in the low shear viscosity of the gel.During this period, erucyl alcohol appears to act as a co-surfactantwhich modifies the micelle structure such that the gel strengthincreases. This initial phase is followed by a progressive decrease inboth the low and high shear viscosity to the point that the fullydegraded fluid has a near-Newtonian viscosity around 8 cP.

EXAMPLE 4 Aqueous Viscoelastic Fluid Wherein the Second Surfactant isAMMONYX GA-90™

[0097] On FIG. 11 is plotted the viscosity of aqueous gels comprising 4wt % of a fluid (cationic surfactant) comprising 60.5 wt %N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride, isopropanoland ethylene glycol, 4 wt % KCl and 0 or 0.1 wt % AMMONYX GA-90™ under ashear of 1 or 100 s⁻¹, as a function of temperature and at pH equal to6.3. AMMONYX GA-90™ is reasonably compatible with the gel. In thepresence of 0.1 wt % of AMMONYX GA-90™, the gel has a lower viscosity inthe temperature range up to 160° F. (71° C.) but a higher viscosity inthe range 176-194° F. (80-90° C.). At high temperatures however, AMMONYXGA-90™ decomposes to the more hydrophilic tri(hydroxyethyl)methylamoniumion and palmitic acid. At the same time, the fluid pH evolves from 6.3to around 3 and, under this acidic condition, palmitic acid is ahydrophobic species which efficiently breaks the gel.

[0098] On FIG. 12 is plotted the viscosity of the above gel comprising0.1 wt % AMMONYX GA-90™ as a function of time, when the formulation isaged at 60 and 70° C. It is observed that the time to a viscosity <1000cP (at 1 s⁻¹) decreases from 15 to 5 hours when the temperature isincreased from 60 to 70° C.

EXAMPLE 5 Aqueous Viscoelastic Fluid Wherein the First Surfactant is aDimeric Oleic Acid

[0099] On FIG. 13 is plotted the viscosity of aqueous fluids comprising4 wt % dimeric oleic acid, 6 wt % KCl and 0, 0.05, 0.1, 0.2 or 0.5 wt %oleyl alcohol, at 60° C. and for a pH equal to 13. The dimeric oleicacid used in the present example and in example 6 is coded U1009 byUnichema International, Bebington, Wirral, Merseyside, United Kingdom.At a high pH, this dimeric acid is converted to carboxylate anions. Inthe FIG. 13, it appears that increasing concentrations of oleyl alcoholfacilitate the breaking of the fluid. However, by comparison with thedata shown in the FIG. 7, it appears that the present gel has a highertolerance to the presence of oleyl alcohol such that more than 0.5 wt %of oleyl alcohol is required to fully break said gel at 60° C.

[0100] Other experiments have been made which show that the viscoelasticproperties of gels based on potassium oleate, monomer, dimer or trimer,are highly sensitive to fluid pH. Typically, when the pH of the fluid isless than 11, the gel is weak and its viscosity is lost at a pH ≦10.5.This behaviour offers another route in terms of the design of a delayedinternal breaker which can slowly degrades to reduce the fluid pH.

EXAMPLE 6 Aqueous Viscoelastic Fluid Wherein the Second Surfactant isMono-Oleyl Ester Succinate

[0101] On FIG. 14 is plotted the low shear viscosity of aqueous fluidscomprising 3.375 wt % of dimeric oleic acid as a function of chlorideconcentration added as KCl and fluid pH, at 40, 60, 70 or 80° C. Theviscosity of the fluids appears to be maximal at a given chlorideconcentration comprised between 0.9 and 1.2 molar.

[0102] On FIG. 15 is plotted the viscosity at salt peak of a fluidcomprising 3.375 wt % of dimeric oleic acid, 6 wt % KCl as a function ofthe temperature, for a pH equal to 9.4 or 11.6, under a shear rate of0.1 or 1 s⁻¹. A decrease in fluid pH results in a considerable decreasein the gel strength and viscosity of salt-optimised gels based on thedimeric oleic acid.

[0103] On FIG. 16 is plotted the viscosity of aqueous fluids comprising4 wt % dimeric oleic acid, 6 wt % KCl and 0.5 wt % oleyl ester succinateas a function of shear rate, at 0, 45, 145 or 190 hours, for an initialpH of 11.5 and at 60° C. The rheology of the formulation evolves from aviscoelastic gel with low shear viscosity between 4800 and 4600 cP and ahigh shear viscosity between 588 cP to a low viscosity solution withnear-Newtonian viscosity around 20 cP.

[0104] When used as an internal delayed breaker for an oleateviscoelastic surfactant system, both the release of oleyl alcohol andthe concomitant decrease in fluid pH serve to break the oleate gel. Theefficiency with which the mono-oleyl succinate breaker can reduce the pHof the oleate gel depends on its initial concentration and the initialpH of the formulation. When added at an initial concentration of 0.5 wt%, the cleavable surfactant can reduce the pH of a typical oleate fluidfrom 11.5 to 9.2. If the initial pH is greater than 12 and so, if theinitial concentration of hydroxide is similar to or higher than theinitial concentration of mono-oleyl succinate, then the gel is brokendown too rapidly for the application. This rapid gel degradation can bealmost instantaneous even at ambient surface temperature. Therefore,according to the invention, the initial pH condition is advantageouslycontrolled.

EXAMPLE 7 Aqueous Viscoelastic Fluid Wherein the Second Surfactants areSulphosuccinates or Sulphoacetates

[0105]FIG. 17 compares the viscosity of aqueous fluids comprising 2 wt %mono-oleic acid, 4 wt % KCl with such fluids further comprising 0.2 wt %active cleavable surfactant added in the form of STEPAN-MILD LSB™ orSTEPAN-MILD SL3™, as a function of shear rate, at 50° C. At a high pH,mono-oleic acid is converted to mono-oleate. The cleavable surfactantscontained in STEPAN-MILD LSB™ and STEPAN-MILD SL3™ appear to becompatible with the dimeric oleic acid viscoelastic surfactant gel.However, the compatibility of STEPAN-MILD LSB™, containing both thesulphosuccinate and sulphoacetate surfactants, is greater than thecompatibility of STEPAN-MILD SL3™ containing only the sulphosuccinatesurfactant. Also, both surfactants induce a significant decrease in thelow and high shear viscosity of the aqueous fluid.

[0106]FIG. 18 compares the viscosity of aqueous fluids comprising 2% wtmono-oleic acid, 4 wt % KCl and 0.2 wt % active cleavable surfactantadded as STEPAN-MILD LSB™ as a function of shear rate, for various timeswhen the fluid is aged at a constant temperature of 50° C. The initialpH is 11.7 and the final pH, at 7.5 h, is 9.7. A systematic decrease inthe low and high shear rate viscosity is observed during the 7.5 hourageing period. After 7.5 hours, the gel has been degraded to a fluidwith near-Newtonian viscosity of about 20 cP.

[0107] On FIG. 19 is plotted the viscosity of aqueous fluids comprising2 wt % mono-oleic acid, 4 wt % KCl and 0.1, 0.2 or 0.5 wt % activecleavable surfactant STEPAN-MILD LSB™ as a function of time, at 50° C.The dosage of cleavable surfactant breaker affects the rate at which thelow shear viscosity of the gel degrades. The data indicate that therange of gel degradation kinetics is appropriate for the application andfor a given initial pH condition, the breaker dosage can be used tocontrol the rate.

[0108] A simpler way to describe the relationship between breaker dosageand gel breakdown rate is shown in FIG. 20 where the x-axis is theconcentration of active surfactant added as STEPAN-MILD LSB™ and they-axis is 1/t, where t is the time required for the gel to lose 90% ofits original viscosity at 1 s⁻¹. The linear relationship shown in FIG.20 is valid for the constant ageing temperature 50° C.

[0109] On FIG. 21 is plotted the viscosity at a shear rate of 100 s⁻¹ ofa gel comprising 2 wt % mono-oleic acid, 4 wt % KCl and 0.5 wt % or 0.2wt % active STEPAN-MILD LSB™, as a function of time, for the followingtemperatures: 50, 60, 70° C. At 50° C., the gel containing 0.5 wt %STEPAN-MILD LSB™ is broken in about 3 hours. At 60° C. it is broken inabout 1.5 hour and at 70° C., it is broken in about 1 hour.

[0110]FIG. 22 compares the delayed breaker performance of the twoSTEPAN-MILD LSB™ and STEPAN-MILD LS3™. The comparison is made at aconstant active surfactant concentration of 0.5 wt % using the same gelformulation and initial pH aged at 50° C. The initial viscosity of thesystem with STEPAN LS3™, is significantly lower than that of the systemwith STEPAN LSB™. The fluid comprising STEPAN-MILD LS3™ may degradefaster than that the fluid comprising STEPAN-MILD LSB™. The data alsosuggest that a more efficient cleavable breaker system for mono-oleicacid may be given by the use of a product containing only laurylsulphoacetate.

1. Aqueous viscoelastic fluid for use in the recovery of hydrocarbons,comprising: a first surfactant, said surfactant being viscoelastic; asecond surfactant, said second surfactant being able to decompose underdownhole conditions to release a compound, said compound being able toreduce the viscosity of the aqueous viscoelastic fluid.
 2. The fluid ofclaim 1 wherein the first surfactant is of the following formulae: R-Zwhere r is the hydrophobic tail of the surfactant, which is a fully orpartially saturated, linear or branched hydrocarbon chain of at least 18carbon atoms and z is the head group of the surfactant which is —NT₁R₂R₃⁺, —SO₃ ⁻, —COO⁻ or, in the case where the surfactant is zwitterionic,—N⁺(R₁R₂R₃—COO⁻) where R₁, R₂ and R₃ are each independently hydrogen ora fully or partially saturated, linear or branched, aliphatic chain ofat least one carbon atom, possibly comprising a hydroxyl terminal group.3. The fluid of claim 1 wherein the first surfactant is cleavable. 4.The fluid of claim 1 wherein the first viscoelastic surfactant isN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride.
 5. Thefluid of claim 1 wherein the first surfactant is a mono-, di- oroligomeric carboxylate.
 6. The fluid of claim 5 wherein the firstsurfactant is oleate.
 7. The fluid of claim 1 wherein the secondsurfactant is a viscoelastic surfactant.
 8. The fluid of claim 1 whereinthe second surfactant is of the following formulae: R—X—Y-Z where R isthe hydrophobic tail of the surfactant, which is a fully or partiallysaturated, linear or branched hydrocarbon chain of at least 18 carbonatoms, X is the cleavable or degradable group of the surfactant which isan acetal, amide, ether or ester bond, Y is a spacer group which isconstituted by a short saturated or partially saturated hydrocarbonchain of n carbon atoms where n is at least equal to 1, preferably 2and, when n is ≧3, it may be a straight or branched alkyl chain, and Zis the hydrophilic head group of the surfactant which is —NR₁R₂R₃ ⁺,—SO₃ ⁻, —COO⁻ or, in the case where the surfactant is zwitterionic,—N⁺(R₁R₂R₃—COO⁻) where R₁, R₂ and R₃ are each independently hydrogen ora fully or partially saturated, linear or branched, aliphatic chain ofat least one carbon atom, possibly comprising a hydroxyl terminal group.9. The fluid of claim 8 wherein the second surfactant is an estercarboxylate, an ester sulphonate, an esterquat, a reverse amidecarboxylate, a forward amide carboxylate, a reverse amide sulphonate, aforward amide sulphonate, a reverse amide quat or a forward amide quat.10. The fluid of claim 9 wherein the second surfactant is a mono- or adiesterquat.
 11. The fluid of claim 10 wherein the esterquat is abetaine ester.
 12. The fluid of claim 10 wherein the esterquat isdi(palmitoylethyl) hydroxyethylmethylammonium or erucyl ester methylenedimethyl ethyl ammonium.
 13. The fluid of claim 9 wherein the secondsurfactant is mono-oleyl ester succinate.
 14. The fluid of claim 9wherein the second surfactant is a sulphosuccinate or a sulphoacetatesurfactant.
 15. The fluid of claim 14 wherein the second surfactant isdisodium laureth sulphosuccinate or sodium lauryl sulphoacetate.
 16. Thefluid of claim 1 wherein the compound is an alcohol, an amine or acarboxylic acid.
 17. The fluid of claim 16 wherein the alcohol, theamine or the carboxylic acid comprise an alkyl chain of at least 3carbon atoms.
 18. The fluid of claim 16 wherein the alcohol, amine orcarboxylic acid are long chain alcohol, amine or carboxylic acidcomprising 8 to 18 carbon atoms or more.
 19. The fluid of claim 16wherein the alcohol is oleyl alcohol.
 20. Method for use in the recoveryof hydrocarbons comprising the following steps: providing an aqueousviscoelastic fluid comprising a first surfactant, said first surfactantbeing viscoelastic, and a second surfactant able to decompose underdownhole conditions; allowing said second surfactant to decompose underdownhole conditions to release a compound able to reduce the viscosityof the aqueous viscoelastic fluid; and allowing the viscosity of thefluid to be reduced downhole.
 21. Method of claim 20 wherein thecompound released reduces the pH of the viscoelastic fluid, saidreduction of pH facilitating the reduction of the viscosity of saidfluid.